Greetings from Alabama

New to this forum and kinda new to bicycles. I've gotten too old to run, so have taken up the bicycle.

Please don't laugh too hard, but I bought a Diamondback Edgewood LX "hybrid" (first bicycle in over fifty years) and am using it three times a week in an exercise program. Presently, I am doing twenty miles per each of three days for a total of sixty miles per week.

Keeping my fingers crossed that I can keep that program going. I am almost seventy years old, heavier than I want to be, and with controlled hypertension. The advice I seek, other than buy another kind of bicycle, is what more should I be doing? More miles? Faster times for the distance (I currently do the twenty miles in an hour and thirty-odd minutes)? Or something else? All of my riding is on the road as there aren't any good trails close by.